The present invention relates to an improvement of non-aqueous solvents used for non-aqueous electrolytes of non-aqueous electrolyte secondary batteries, and particularly to an improvement of charge and discharge characteristics in a low-temperature environment.
Recently, electronic devices such as personal computers and portable telephones of miniature and light-weight type or cordless type have been rapidly developed, and secondary batteries having high energy density have been demanded as electric sources for driving these devices. Among them, non-aqueous electrolyte secondary batteries using lithium as an active material are expected much as batteries having high voltage and high energy density. Hitherto, in these batteries, metallic lithium is used for the negative electrode and molybdenum disulfide, manganese dioxide, vanadium pentoxide or the like is used for the positive electrode, and batteries on the level of 3 V have been realized.
However, when metallic lithium is used for the negative electrode, dendrite lithium is precipitated during charging, and the dendrite lithium deposited on the electrode plate is released from the electrode plate with repetition of charging and discharging, is suspended in the electrolyte, and contacts with the positive electrode to cause a minute short-circuit. As a result, the charging and discharging efficiency becomes lower than 100%, and the cycle life is shortened. Moreover, the dendrite lithium is large in surface area and high in reaction activity, and, hence, has a problem in safety.
In an attempt to solve these problems, recently, researches have been intensively conducted on lithium ion secondary batteries in which a carbon material is used in place of the metallic lithium and a lithium-containing transition metal oxide which shows a voltage of 4 V level for lithium, such as LiCoO2, LiNiO2 and LiMn2O4, or the like is used for positive electrode, and some of them have already been marketed. In these batteries, since lithium is present in the state of being absorbed in carbon in the negative electrode, the dendrite lithium seen in the conventional negative electrode using metallic lithium is not precipitated and safety is markedly improved.
As mentioned above, in non-aqueous electrolyte secondary batteries, especially, lithium ion secondary batteries, characteristics of positive electrode and negative electrode are naturally important, but characteristics of non-aqueous electrolyte which carries lithium ion are also important for obtaining satisfactory characteristics of the batteries. As non-aqueous solvents constituting the non-aqueous electrolytes, ordinarily, a solvent of high dielectric constant which is high in dissolvability for electrolyte and a solvent of low viscosity which is high in carrying ability of electrolyte ion are used in combination. For example, high conductivity can be provided by electrolytes comprising a mixture of cyclic carbonic acid esters such as ethylene carbonate (hereinafter sometimes referred to as xe2x80x9cECxe2x80x9d) and propylene carbonate (hereinafter sometimes referred to as xe2x80x9cPCxe2x80x9d) which are solvents of high dielectric constant and non-cyclic carbonic acid esters such as dimethyl carbonate (hereinafter sometimes referred to as xe2x80x9cDMCxe2x80x9d), diethyl carbonate (hereinafter sometimes referred to as xe2x80x9cDECxe2x80x9d) and ethyl methyl carbonate (hereinafter sometimes referred to as xe2x80x9cEMCxe2x80x9d), and these electrolytes have been widely used.
However, EC has a high freezing point of about 38xc2x0 C. , and when it is used alone, the freezing point decreases to about 0xc2x0 C. at the lowest even in expectation of freezing point depression caused by mixing with a solute. Therefore, it is attempted to ensure the low-temperature characteristics by mixing with a solvent of low viscosity and low freezing point. However, this mixed solvent contains EC and is affected by EC not a little, and thus sufficient low-temperature characteristics cannot still be ensured. Then, there is proposed an electrolyte using PC which is another cyclic carbonic acid ester having a low freezing point of xe2x88x9249xc2x0 C. and a high dielectric constant. Although this electrolyte is improved in low-temperature characteristics as compared with the electrolyte using EC, it is still insufficient in low-temperature characteristics even if it is used in admixture with other solvents. Furthermore, when it is used in batteries in which a graphite of high crystallinity is used for negative electrode, there is a problem that PC is decomposed with this graphite.
Furthermore, cyclic carboxylic acid esters are proposed as solvents of high dielectric constant substitutable for the cyclic carbonic acid esters. As the cyclic carboxylic acid esters, for example, xcex3-butyrolactone has a low freezing point of xe2x88x9245xc2x0 C. like PC and has a high dielectric constant, and, besides, is much higher than PC in conductivity at low temperatures. Therefore, this solvent is a very preferable solvent for lithium batteries. However, cyclic carboxylic acid esters readily undergo reductive decomposition and decomposes at a potential of negative electrode during charging when materials of low potential such as graphite are used for negative electrode. Thus, there are problems that irreversible capacity increases and charging and discharging efficiency decreases.
The object of the present invention is to solve the above problems and provide non-aqueous electrolyte secondary batteries excellent especially in charging and discharging characteristics at low temperatures.
As a result of intensive research conducted by the inventors, it has been found that an electrolyte applicable to non-aqueous electrolyte secondary batteries and giving excellent low-temperature characteristics can be obtained by adding to a cyclic carboxylic acid ester a cyclic carbonic acid ester having at least one carbon-carbon unsaturated bond.
That is, the present invention is a non-aqueous electrolyte secondary battery containing a positive electrode, a negative electrode and a non-aqueous electrolyte where the non-aqueous electrolyte contains a solute and a non-aqueous solvent, and the non-aqueous solvent contains a cyclic carboxylic acid ester and a cyclic carbonic acid ester having at least one carbon-carbon unsaturated bond.
The inventors consider that the excellent low-temperature characteristics according to the present invention are obtained for the following reasons, although they do not intend to suffer restriction by a specific theory.
According to Aurbach et al, xe2x80x9cJ. Electrochem. Soc., 138, 3529) and others, it is considered that a cyclic carbonic acid ester such as EC undergoes ring-opening and is dimerized at the time of reduction, thereby forming a film (passive state layer) on the surface of negative electrode, and this film acts as a physical barrier which inhibits insertion of solvent molecules around lithium ions. The above literature makes no mention of cyclic carbonic acid esters having unsaturated bond.
JP-A-11-31525 reports an electrolyte containing xcex3-butyrolactone (hereinafter sometimes referred to as xe2x80x9cGBLxe2x80x9d) to which EC is added. However, according to the experiments conducted by the inventors, it has been found that this electrolyte though containing EC does not form the effective physical barrier reported by Aurbach et al.
As a result of investigations conducted by the inventors on various materials, it has been found that it is preferred to add not the cyclic carbonic acid esters having no carbon-carbon unsaturated bond, such as EC, but the cyclic carbonic acid esters having carbon-carbon unsaturated bond, such as vinylene carbonate (hereinafter referred to as xe2x80x9cVCxe2x80x9d) to cyclic carboxylic acid esters such as GBL which readily undergo reductive decomposition. In the case of these esters having carbon-carbon unsaturated bond, polymerization first occurs at the unsaturated bond site and then the ring-opening dimerization seen in the case of EC proceeds, and, hence, it is considered that the film formed on the surface of negative electrode becomes denser and stronger than in the case of using EC. That is, it is considered that the effective physical barrier is formed to effectively inhibit the reductive decomposition of cyclic carboxylic acid esters such as GBL.
Examples of adding VC to electrolytes are reported in JP-A-6-84542 and JP-A-8-45545, but, in these examples, VC is added to cyclic carbonic acid esters having a high freezing point, such as EC, and they do not disclose addition of VC to cyclic carboxylic acid esters.
As mentioned above, according to the present invention, non-aqueous electrolyte secondary batteries having a very high conductivity even at low temperatures, especially excellent in charging and discharging characteristics in low-temperature environment can be provided by constituting the electrolyte by adding cyclic carbonic acid esters having carbon-carbon unsaturated bond to cyclic carboxylic acid esters. Particularly, even in the batteries using for negative electrode a graphite which cannot hitherto be used because it decomposes cyclic carboxylic acid esters, the electrolytes containing cyclic carboxylic acid esters can be used and improvement of low-temperature characteristics can be attained.